A boiler is a significant part of an investment in a ship, a manufacturing plant, a commercial facility or the like. Accordingly, when such a facility is idle, some effort is often made to prevent the loss of this investment to corrosion. The addition of an oxygen scavenger (reducing agent) and an alkalizing agent to the water of an inactive boiler is one way to inhibit internal corrosion. Hydrazine is a common reducing agent used as a chemical oxygen scavenger to protect against corrosion
Typically, inactive boilers are ignored because they do not pose any immediate safety hazard or other risk. In the marine industry, there are vessels that are “mothballed” for extended periods at a time. Such vessels are left unattended, except for a watchman making his or her rounds. Historically, when the vessel's boilers are laid up wet (full of water), nobody monitors the boiler water chemistry for months at a time. Overtime air (oxygen) enters the water system of the boiler and depletes the reducing agent added to the system.
Eventually, corrosion will occur and remain undiscovered until the boiler is placed back on line. By then there may have been sufficient corrosion to cause failure of boiler tubes under operating pressures. Such failures may require substantial time to repair, during which time the vessel must remain out of service or be substantially limited in its service capability. Such repairs are costly and time consuming. Furthermore, if they occur while the vessel is at sea, the vessel, its cargo, and its personnel may be at risk, particularly if the vessel becomes dead in the water.
Hydrazine has been used as a chemical oxygen scavenger to protect against corrosion caused by oxygen in laid up boilers for over 30 years, but the hydrazine is administered to the boiler manually. Manual treatment leaves much to be desired. The hydrazine is either dumped into an expansion tank with distilled, de-ionized, or a mixture with shore water (oxygen saturated) and then water is drained out of the bottom blowdown to allow the chemical treatment to enter the boiler, or the treatment is injected into the feedwater line with a chemical pump. When the former method is used, much of the hydrazine is wasted because the treatment contains fresh water saturated with oxygen, which depletes the oxygen scavenging effect of the hydrazine. When the latter method is used, the feedwater pump must be run to transport the treatment into the boiler and some of the boiler water will have to be drained out to make room. In both cases, the secondary disadvantage is the change in boiler water chemistry that results from the fresh water.
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